1. Field of the Invention
This invention relates to a process for direct shaping and optimization of the mechanical characteristics of penetrating projectiles of high-density tungsten alloys, in particular projectiles for military ammunition.
2. Description of the Background
Penetrating projectiles which are used in military weapons have undergone considerable development in recent years. The use of alloys of increasing density, with the objective of optimizing the mechanical characteristics thereof, in combination with an increase in the rate of fire, has made it possible to produce increasingly effective projectiles.
Alloys which thus far have been developed included:
Alloys based on depleted uranium, with which it is possible to achieve a density of close to 19,000 kg/m.sup.2 and good ductility. The use of such alloys is made attractive by the need to find outlets for the stocks of depleted uranium which are generated by the nuclear industry;
Tungsten carbide containing about 13% to 15% of cobalt. This alloy, however, suffers from the disadvantage of having a density of 14,000 kg/cm.sup.3, which is insufficient for certain uses. Moreover its low level of ductility can be a handicap from the point of view of piercing multiple targets;
Tungsten-based alloys which are produced by powder metallurgy. The tungsten used in the preparation of such alloys contains the usual impurities, the alloy exhibits low ductility and the machining of the alloy is delicate, both of which factors are impediments to its use. Other alloys of tungsten with, for example, nickel, copper and iron, resulting in alloys of the W-Ni-Cu and W-Ni-Fe type, are such that the properties of the alloys can be relatively well controlled depending upon the use of the alloy. For example, in the case of W-Ni-Cu alloys which have a density of between approximately 17,500 and 18,500 kg/m.sup.3, the same have a mean ductility which is an attractive feature from the point of view of the fragmentation of the projectile. In the case of W-Ni-Fe alloys, whose density can also be adjusted to between 17,500 and 18,500 kg/m.sup.3 by varying the tungsten content (93% to 97% by weight), the ductility of these alloys can be modified as a function of the Fe/Ni ratio.
The production of W-Ni-Cu and W-Ni-Fe alloys which are also referred to as "heavy metals" is accomplished by powder metallurgy. The raw materials are used as powders of each of the metals having a granulometry of between about 2 and 10 .mu.m. The powders are mixed in rotary apparatuses, in particular, thereby producing a homogeneous product, the analysis of which corresponds to the desired composition. The mixture is then formed into the form of blanks of a profile which is suitable for the required use, either by a compression operation in a steel shaping die or by isostatic compression, in the course of which the powder which is placed in a rubber mold is subjected to the action of a compression fluid in an enclosure at high pressure. The blanks produced are porous, of low density and fragile and they have to be subjected to a densification operation which is effected by sintering at a temperature approximately between 1400.degree. and 1600.degree. C. in furnaces in a hydrogen atmosphere. In the course of densification a ternary phase formed by the three metals involved is formed by diffusion and becomes liquid. That liquid encases the grains of tungsten and permits complete densification of the alloy by a substantial dimensional contraction of the blank.
The alloys based on tungsten metal, the process for the production of which has just been described above, may exhibit ductility. By virtue of this property, it is possible to improve their elastic limit and their breaking stress, by a working operation.
Thus, for example, a blank made from an alloy containing by weight 93% W, 4.5% Ni and 2.5% Fe, after sintering at 1450.degree. C., has the following characteristics:
density: 17,500 kg/m.sup.3 PA0 resistance to 0.2% elongation Rp 0.2: 750 MPa PA0 breaking strength Rm: 950 MPa PA0 elongation: 25%. PA0 Rp 0.2: 1100 MPa PA0 Rm : 1250 MPa.
After homogeneous working of the blank at a rate of reduction in section of about 18%, the blank has the following strength values:
A work-hardened material of this kind is used to produce subcaliber projectiles intended for piercing armour plating as it has a high elastic limit capable of withstanding the stresses due to acceleration in the gun in which the muzzle velocities can attain 1400 to 1600 m/sec. When the blank is to be worked to produce such projectiles, the blank is generally a cylindrical shape and the working operation is hammering in a moving mode. In order to impart the definitive profile of the projectile to the blank, it is then subjected to a suitable machining operation.
A process of that kind is described in U.S. Pat. No. 3,979,234. It is stated therein that projectiles of W-Ni-Fe alloy of the composition by weight of 85-90% W, with the Ni/Fe ratio ranging from 5.5 and 8.2, are produced by powder compression, sintering, working the blank at a rate of reduction of 20% and then final machining of the worked blank. By this process it is possible to achieve a Rockwell hardness of 42, which is uniform to within plus or minus one unit.
It should be noted however that such a process suffers from two major disadvantages:
On the one hand, the operations of machining the blank after sintering and after working result in a relatively substantial loss of expensive material, which has a serious adverse effect on the cost price of the projectiles, not to mention the labor costs that it involves:
On the other hand, homogeneity of the properties of the projectiles is not always desirable. In fact, projectiles are subjected to different forces acting thereon during their use which include:
(i) mechanical shock stresses when the projectiles are loaded at a high rate into the barrel of the gun; PA1 (ii) very high elastic stresses during the phase of acceleration in the gun; and PA1 (iii) various stresses upon impact against the target which may be composed of layers of different materials, causing the phenomena of compression, working and increase in temperatures.
Moreover, it is desirable that, in the final phase of penetration of a target, the projectiles fragment in order to increase their destructive capacity.
For all those reasons, it is desirable to provide projectiles which are constituted of zones with different metallurgical characteristics which are optimized in such a way as best to comply with the specific forces to which they will be locally subjected. A need therefore continues to exist for a process of forming penetrating projectiles which remedies the two disadvantage referred to above.